Clear Iceland spar (calcite), cleaved, optical effect

A lovely clear crystal of the Iceland spar form of calcite, which has been cleaved into its rhombic form.  If you look at something through this, you get a double image!  This specimen is from Minas Gerais, Brazil.  The specimen is discreetly attached to a black plastic base.

3/4 inch (1.9 cm) wide, 5/8 inch (1.6 cm) deep, 3/4 inch (1.9 cm) tall.